Design of a temperature-compensated crystal oscillator using the new digital trimming method

Recently, the demand for stable temperature-compensated crystal oscillators (TCXO) has: been increasing more and more, and digital TCXOs (DTCXO) have been studied extensively because of their higher frequency accuracy and one -chip implementation possibility. A DTCXO using a capacitor array which is directly controlled by a digital code from the memory has been proposed for developing a VLSI TCXO, and the way to organize the capacitor array has be en studied. In this work, a new capacitor array scheme, called TACA (temper ature adaptive capacitor array), is proposed. It guarantees monotonicity an d saves the silicon area at the same time. We also have developed a TCXO th at can be used over a wide range of frequencies. The oscillator and the cap acitor array were fabricated with a 0.5-mum CMOS process and their operatio n was characterized at 10 and 20 MHz. Complete digital trimming of the DTCX O was achieved with a 0.2-ppm trimming accuracy.